Computed Radiography (CR) is a well known technology for recording radiation images which is based on stimulable storage phosphors. As described in U.S. Pat. No. 3,859,527 (Luckey), when certain types of phosphors are exposed to high energy radiation, such as x-rays, gamma rays, etc., they store a portion of the energy of the incident radiation. If the phosphor that has been exposed to high-energy radiation is then exposed to a second, stimulating radiation, such as visible light or heat, the phosphor will emit radiation in proportion to the stored energy of the high energy radiation. Screens formed from such storage phosphors have been discussed in the literature as having very desirable properties, in terms of sensitivity and exposure latitude, for the detection of x-ray images in projection radiography.
The storage phosphor's stimulated signal is recognized as being linearly related to radiation exposure over at least 5 orders of magnitude. It is recognized that it can be challenging to design analog electronics that can handle signals over such a wide range without degradation. Likewise, analog-to-digital converters do not typically cover such a large signal variation. Analog compression schemes, such as logarithmic amplifiers, tend to have speed and gain limitations. Thus, while storage phosphor systems are inherently well suited for projection radiography, it is challenging to design economical electronic systems which do not degrade the available signal.
There are a number of parameters that, taken together, characterize the performance of x-ray imaging systems. For CR, these parameters include spatial resolution, noise, detective quantum efficiency (DQE), exposure response, and artifacts.
For CR, the modulation transfer function (MTF) of the imaging system is often used to characterize the system spatial resolution. MTF is a 2D (two-dimensional) function of spatial frequency and is usually measured for both x and y directions of the acquired image.
The noise of the imaging system determines the system low-contrast resolution as well as the x-ray detective efficiency. The noise characteristics can be described by the noise power spectrum (NPS) of the imaging system, which is also a 2D function of spatial frequency. To obtain the NPS, a flat image region is usually taken for Fourier analysis. Because the system noise level is also x-ray exposure-dependent, the NPS is often measured at a certain exposure level to facilitate comparisons among imaging systems.
Detective quantum efficiency (DQE) is a secondary parameter of the imaging system that can be calculated from the system MTF, NPS, and the air kerma of the x-ray exposure at the detector.
It is desirable to have an x-ray imaging system with improved MTF and DQE to provide improved diagnostic efficacy and/or lower patient dose. However, factors that tend to improve high frequency MTF tend, at the same time, to degrade low frequency DQE. Similarly, steps taken to improve low frequency DQE tend to compromise high frequency MTF. For example, for a given storage phosphor screen thickness, increasing the readout laser exposure decreases high frequency MTF while increasing low frequency DQE. Conversely, lower laser exposure increases high frequency MTF while decreasing low frequency DQE. Given these constraints, optimizing CR by simultaneously improving MTF and DQE is challenging.
There has been some effort expended for reading out wide dynamic range images with storage phosphor systems. One technique is the use of a preliminary scan at low stimulating intensity to determine the exposure level of the latent image on the storage phosphor screen. For example, U.S. Pat. No. 4,527,060, issued Jul. 2, 1985 (Suzuki et al.) reads a small percentage of the latent image using a low power stimulating beam, and uses this information to optimally set the gain or scaling factor of the electronics for a full intensity final scan, to ensure that no information is lost due to too high an exposure or inadequate gain. However, this operation causes some degradation in the DQE of the final scan.
U.S. Pat. No. 4,837,436, issued Jun. 6, 1989 (Whiting), commonly assigned, performs two scans of the image. A first scan of the latent image is conducted at a low stimulating exposure to capture the high x-ray exposure image signal and a second scan is conducted at a high stimulating exposure to capture the low x-ray exposure image signal. Information from both signals is then combined into one wide dynamic range image signal covering a wider dynamic range than could be obtained by a single scan. This approach enhances the dynamic range, but does not improve the system MTF.
Another technique for optimizing the output image data is to conduct two scans of the image. U.S. Patent Application Publication No. 2003/0020031 entitled “Radiation Image Read-out Method and Apparatus” by Otokuni describes an image plate reading mechanism that, using a movable read-out device, obtains a first reading of the stored image when scanning in one direction and a second reading that obtains residual image data when scanning in the opposite direction. However, this approach does not improve the system MTF.
U.S. Patent Application Publication No. 2006/0091338 entitled “Image Acquisition System for Improved DQE” by Koren describes a scanning method using first and second laser beams from a beam direction apparatus rotatable on an axis.
Another approach to improve image quality is the use of a dual-side read technique for CR image plates. With this method, described, for example, in U.S. Pat. No. 5,877,508 entitled “Radiation Image Storage Panel” to Arakawa et al., the stored image is obtained from read-out sensors that are positioned on opposite sides of the imaging plate.
Imaging panels having multiple phosphor layers have been proposed, for example, see U.S. Pat. No. 6,479,834 entitled “Double-Sided Reading System for Reproducing Radiation Image” to Suzuki. Approaches for imaging panel optimization have included the use of multiple phosphor layers, wherein the phosphors have different particle sizes on each layer, as described in the Suzuki '834 patent.
Layers on the same imaging plate and having different thickness have also been proposed for obtaining different energy levels of x-ray radiation, as described in U.S. Patent Application Publication No. 2006/0180773 entitled “Radiography System and Method for Recording X-Rays in Phosphor Layers” by Frankenberger et al.
The dual-sided scan approach can improve low-frequency DQE, but does not improve the system MTF.
Other approaches have included use of colorant layers for optimizing the obtained output signal by selective absorption of various wavelengths, as described, for example, in U.S. Pat. No. 4,380,702 entitled “Radiation Image Storage Panel” to Takahashi et al. The use of colorant particles dispersed within one or more phosphor layers is also described as an optimization technique in U.S. Pat. No. 5,591,982 entitled “Radiation Image Storage Panel and Radiation Image Recording and Reproducing Method” to Kohda. Laser stimulation can be directed to the dual-sided phosphor layers by a single laser on one side of the imaging plate or by lasers on opposite sides of the imaging plate, for example as described in U.S. Pat. No. 6,016,356 entitled “Image Superposition Processing Method” to Ito et al. The pixel image data for the final image from these systems is obtained by combining the superimposed data components from each sensor. Various methods have been proposed for this combination, typically using some type of weighted addition technique, with various more elaborate processing techniques such as Fourier transform processing and wavelet transform processing also described.
While there have been attempts to optimize the read-out apparatus, imaging panel design, and image combination algorithms, the dual-sided read approach is hampered by a number of difficulties inherent to this image-reading method. For example, the read-out apparatus positioned on each side of an imaging plate adds bulk and complexity to the design of an image-reading device. Moreover, as noted earlier, neither the dual-sided read approach nor proposed approaches using multiple scans improve system MTF.
Thus, while conventional CR x-ray imaging systems may have achieved a certain degrees of success in their particular applications, there still exists a need to provide a CR imaging system with improved image quality. Because the potential benefits of improved image quality can help both to enhance diagnostic efficacy and to reduce patient dosage levels, there is high motivation for achieving even incremental improvements in performance.